U. Glaeser

FIGURE 39.70
FIGURE 39.71
efficient to implement them in hardware, especially if multiple filter types can use the same filter structure.
Moreover, while FIR and IIR filter structures are quite different, a clever designer may find opportunities
to reuse the same basic arithmetic hardware for both. For example, both structures can use a multiplyaccumulate
arithmetic unit. If each structure requires only one-half the available bandwidth, they can
share the same math unit by time-division multiplexing the inputs. Because arithmetic units are generally
costly, techniques such as this can significantly reduce the system cost.
Effects
Music synthesis applications use effects such as reverb, delay, and chorus as sweeteners. They are not
required, but tend to make the music sound more pleasing. In contrast, 3-D positional and environmental
simulation applications require delay and reverb to achieve realistic results. The fundamental unit of many
digital audio effects is the digital delay line. A simple echo effect may use only one delay line, whereas a
reverb effect may use 20 or more delay lines. Even modulation effects such as chorus and flange use delay
lines. Digital delay lines require one memory location per sample period of delay, an address generator,
and arithmetic units to scale the inputs and outputs of the delay line. Figure 39.71 illustrates a typical
delay line implementing a repeating echo through the use of feedback.
An obvious method of implementing a set of delay lines is to allocate memory buffers for each delay
line and maintain circular address counters for each indicating the read and write locations. The delay
time is equal to the difference between the read and write pointers, modulo buffer size. Maintaining
circular address counters can easily use a large percentage of the total instruction bandwidth of a DSP.
Many DSP implementations provide special instructions or self-maintaining address registers to reduce
the load on the DSP.
To provide maximum flexibility in implementation of effects, most PC audio systems include a programmable
DSP. The designer may choose to purchase an off-the-shelf DSP, either as a separate chip or
© 2002 by CRC Press LLC
Bass and treble tone control filter shapes.
Input
Low Shelving
Filter (Bass)
Delay Memory
Feedback Amount
High Shelving
Filter (Treble)
Delay line with feedback implementing a repeating echo.
N
N
freq
freq
Output